1. Field of the invention
The present invention relates to a plastic sheet which has creasing lines and is formed into a packaging container or the like.
2. Description of the Related Art
When a packaging container or casing is formed through bending of a plastic sheet, a groove-like creasing line is press-formed at each portion of the plastic sheet at which the plastic sheet is to be bent. Subsequently, the plastic sheet is bent along the creasing lines in order to complete the packaging container or casing. U.S. Pat. No. 5,302,435 discloses such a technique. Conventional plastic sheets have employed creasing lines of various shapes and structures. The structure of such a creasing line greatly affects the performance of a completed plastic container or casing.
When a packaging container is to be formed through bending a plastic sheet, as shown in FIG. 1, grooves 2 are formed on a sheet 1 punched into a planar shape corresponding to the shape of the container, and the sheet 1 is then bent along the grooves 2 to thereby complete the container as shown in FIG. 2. The grooves 2 are called xe2x80x9ccreasing linesxe2x80x9d or simply xe2x80x9ccreasing lines.xe2x80x9d The creasing lines are formed by a process such that a member called a xe2x80x9ccreasing line-forming bladexe2x80x9d is pressed against the sheet 1.
The technique for bending a sheet after formation of creasing lines by use of creasing line-forming blade has conventionally been used for fabrication of paper containers. However, when this technique is applied to fabrication of plastic containers, fabrication of containers having corners of a desired angle is difficult, because a plastic sheet has higher resistance against bending and higher elasticity that does a paper sheet.
Various techniques for solving the above-described problems have been proposed. One solution is employment of creasing lines having a special shape to thereby facilitate bending operation. For example, Japanese Utility Model Publication (kokoku) No. 4-9345 discloses a plastic sheet having creasing lines each formed of a groove in which projections and depressions are formed alternately on the bottom surface along the longitudinal direction thereof. Japanese Patent Application Laid-Open (kokai) No. 64-40317 discloses a plastic sheet having creasing lines each formed of a groove in which holes are formed in the bottom portion at a predetermined pitch along the longitudinal direction thereof.
In relation to a method of bending a plastic sheet, Japanese Patent Application Laid-Open (kokai) No. 2-98422 discloses a method in which a plastic sheet having creasing lines is first folded at each of the creasing lines, then unfolded to the original state, and then subjected to a bending process.
Furthermore, a creasing line-forming blade having an improved structure has been proposed. Japanese Patent Application Laid-Open (kokai) No. 1-141720 (Japanese Patent No. 2541252) discloses a creasing line-forming blade for a plastic sheet whose tip end has concave portions and convex portions arranged alternately along the longitudinal direction, wherein the concave portion has a length of 0.3 to 2 mm, the convex portion has a length of 0.02 to 0.15 mm, the tip end of the convex portion has a width not greater than 0.5 mm, and the tip angle is 30 to 130xc2x0.
A recent automatic container fabrication machine can perform, at high speed, a series of operations for bending a sheet to complete a three-dimensional container, charging a liquid or the like into the container, and sealing the container. Although the above-described techniques have enabled fabrication of containers through bending of a plastic sheet having creasing lines, when the conventional plastic sheet is used in such an automatic container fabrication machine, there arise various problems such as breakage of a creasing line portion, and failure in formation of a three-dimensional shape.
In a plastic sheet having creasing lines, bending operation is facilitated through a decrease in the thickness (residual thickness) of the plastic sheet at the bottom of each groove serving as a creasing line. However, when the residual thickness is decreased, a strong force tends to act locally at creasing line portions during bending, especially during a step of forming a plastic sheet into a final shape by use of an automatic container fabrication machine, resulting in breakage of the container from a creasing line portion. This problem of breakage becomes remarkable when holes are provided at creasing line portions in order to facilitate a bending operation.
Further, conventional plastic sheets involve a problem in relation to appearance. A packaging container formed of a plastic sheet is more excellent than a paper container in terms of transparency, because the plastic sheet can be transparent. However, when creasing lines having a complicated shape, such as creasing lines having holes as shown in Japanese Patent Application Laid-Open No. 64-40317, are employed in order to facilitate a bending operation, after completion of a container, light scatters at the creasing lines, and corner portions become conspicuous, resulting in possible impairment of the appearance of a transparent container. This is a serious problem because plastic containersxe2x80x94which are more expensive than paper containersxe2x80x94are used in order to improve the appearance of a commodity so as to give the impression of a high-quality article.
As described above, when plastic sheets are used for fabrication of packaging containers, it becomes important to cope with fabrication by use of an automatic container fabrication machine and to form creasing lines which do not impair the appearance of a resultant container.
In view of the foregoing problem, an object of the present invention is to provide a plastic sheet having creasing lines which can cope with fabrication by use of an automatic container fabrication machine and which do not impair the appearance of a resultant container.
The present invention provides a plastic sheet in which each creasing line is formed of a groove having a bottom surface and a pair of opposed side surfaces each slanting at a certain angle; and a plurality of ribs each having a shape of a semi- or half-cylinder (hereinafter referred to as xe2x80x9csemi-cylindrical ribsxe2x80x9d) are formed on the bottom surface at substantially equal intervals along the longitudinal direction such that the ribs connect the opposed side surfaces. Preferably, the area of a portion of the bottom surface not occupied by the ribs accounts for about 90% to about 95% the entire area of the bottom surface as measured when the ribs are not present.
In the plastic sheet having creasing lines of the present invention, the semi-cylindrical ribs connecting the opposed side surfaces provide a reinforcing function. In addition, during a bending operation, stresses concentrate in the vicinity of the semi-cylindrical ribs, and consequently at the creasing lines, thus enabling formation of neat pleat lines.
The present invention further provides a method of fabricating a plastic sheet having creasing lines by use of a creasing line-forming blade. The blade has a flat tip-end surface extending in the width direction and a pair of opposed side surfaces each continuing from the tip-end surface and slanting at a certain angle. Depressions for forming the semi-cylindrical ribs are formed on the tip-end surface at substantially equal intervals. The creasing line-forming blade is pressed against a plastic sheet in order to form a creasing line having a depth 0.6 to 0.9 times the thickness of the sheet.
When the creasing line-forming blade is pressed against a plastic sheet, molecules of plastic within the plastic sheet are densified, and the pressure-subjected portion becomes hard. This increases the stability of a shape obtained through a bending operation. In the plastic sheet of the present invention, since semi-cylindrical ribs are present at substantially constant intervals, the degree of compression and other properties differ between the rib portion and the remaining flat bottom portion. This renders complex the distribution of stress at a bent portion, which conceivably contributes to an increase in shape stability.